An examination of the developmental changes in thyrotropin releasing hormone (TRH) does not correlate with alterations in the rates of hormone degradation in the rat hypothalamus. The rise in TRH content begins approximately four days after birth and is maximal at 20 days, whereas both soluble and particulate rates of TRH degrading change little over the same time period. TRH degradation in serum has been suggested to be of physiological importance by several previous investigations. Our laboratory has partially purified and characterized an enzyme from rat serum which degrades TRH into pGlu and His. Pro. NH2. A number of compounds which might influence the thyroid status of the animal do not affect the rates of TRH degradation by a direct interaction with the enzyme.